MELTDOWN ON THREE MILE ISLAND: What Happened On The Day Of The Nation's Worst Nuclear Disaster

Thirty-four years ago today, the worst nuclear disaster in U.S. history shook the nation to its core.

The drama began at 4 a.m. on Three Mile Island, located in the middle of Susquehanna River, near Harrisburg, Pa..

The island is home to two nuclear reactors. One of them continues to function and deliver power. The second one has not been run again since March 28, 1979, when a few malfunctions and a series of human errors resulted in a partial nuclear meltdown.

About 20 tons of radioactive uranium spilled out of the reactor core and almost burned through the five-inch thick steel floor.

It was not as bad as the disasters at Fukushima or Chernobyl, but a tremendous nuclear catastrophe was narrowly avoided.

The event triggered a public backlash against nuclear energy, and fueled the popularity of a movie called "The China Syndrome."

Here is the story of what happened on that particularly dark day in our nation's history.

The nuclear plant known as Three Mile Island was built on an island of the same name in the middle of the Susquehanna River, about five miles south of Harrisburg, Pennsylvania.

The island had two installations on it. TMI-1 was finished 1974, and it has run ever since with little incident. But TMI-2 was another story.

Centers For Disease Control and Prevention

Unit 2 was newer, but in the words of a Nuclear Regulatory Commission report, had been "bedeviled by a series of mishaps-mostly minor, but troublesome" since it opened.

Department of Energy

Nuclear power basically works like this: In the core of a nuclear plant, splitting atoms create heat. Water pumped into the reactor turns into steam and then escapes through a pipe where it powers a turbine. This is what generates electricity.

But here is the hitch — the water pumped into the reactor also has the effect of preventing the extremely hot nuclear core from overheating. If something goes wrong, and the core overheats, then the plant risks a nuclear meltdown.

NRC Special Inquiry Report

This is exactly what happened at the nuclear power plant on Three Mile Island on March 28, 1979. In 10 short seconds, a clogged pipe and a stuck valve caused the worst nuclear disaster in U.S. history.

At about 4 a.m., either a mechanical or electrical malfunction clogged the pipe that pumped water into the reactor.

Immediately, the heat and pressure inside the reactor started to rise. The control rods on the top of the core shut down the reaction process and a valve popped open to let steam escape out of the system.

Over the next two hours, as water continued to boil out of the core and radiation levels in the building rose, the core grew so hot that at 5:41 a.m. the nuclear fuel rods began to crack and melt.

At 6:18 a.m., engineers finally figured out that the valve was stuck and closed a backup valve to stop the flow. They would not realize that the core might be low on water for another hour. Plant employees had to be checked for radiation after the accident.

By the time they finally did pump water into the core, the fuel rods had melted into a giant mass, which the water could not penetrate. But by that time radiation levels were so high, operators had no choice but to notify the Nuclear Regulatory Commission of a general emergency. Even President Jimmy Carter was notified, and he visited the site a few days later on April 1, 1979.

Twenty tons of molten uranium began spilling out of the core onto the five-inch solid steel floor of the reactor vessel. Unlike the disasters in Chernobyl and Fukushima, the reactor vessel did not crack. By 9 a.m. the worst of the disaster had passed.

Though the accident only leaked a small amount of radiation into the surrounding environment, there was a significant public backlash against the facility and nuclear power in its aftermath. The Nuclear Regulatory Commission did not review new applications for nuclear reactors again until 2012.